1. Field of the Invention
The present invention relates to a zoom lens, and more particularly to a zoom lens suitable for an image pickup apparatus using a solid image pickup element as a image pickup device, such as a video camera or a digital camera.
2. Description of the Related Art
According to the prior art, zoom lenses of the so-called negative lead type, in which the front-most lens unit has negative optical power (optical power is reciprocal of the focal length), are used as standard zoom lenses for many cameras because of their relative ease in widening the field angle.
As a standard zoom lens of this kind, there is proposed in, for instance, the Japanese Patent Laid-Open No. 53-132360 (corresponding to the U.S. Pat. No. 4,299,452), the Japanese Patent Laid-Open No. 56-19022 (corresponding to the U.S. Pat. No. 4,370,031) and the U.S. Pat. No. 5,283,639, a so-called two-unit zoom lens which consists of two lens units including a first lens unit having negative optical power and a second lens unit having positive optical power, and varies the magnification by moving these two lens units along the optical axis and thereby varying the distance between the lens units.
Further in the Japanese Patent Laid-Open No. 7-52256, there is proposed a zoom lens which has three lens units including, in the order from the object side, a first lens unit having negative optical power, a second lens unit having positive optical power and a third lens unit having positive optical power, and varies the magnification from the wide angle end toward the telephoto end by increasing the distance between the second and third lens units.
Also in the U.S. Pat. No. 543,710, there is proposed a zoom lens which has three lens units including, in the order from the object side toward the lens, a first lens unit having negative optical power, a second lens unit having positive optical power and a third lens unit having positive optical power, and varies the magnification from the wide angle end toward the telephoto end by decreasing the distance between the second and third lens units.
Further, in the Japanese Patent Laid-Open No. 6-27377 (corresponding to the U.S. Pat. No. 6,104,548), there is proposed a multi-unit zoom lens comprising three or more lens units.
In these zoom lenses, where a lens unit having a positive optical power is to be composed of a plurality of lenses, usually a glass material having a greater Abbe number for positive lenses and a glass material having a smaller Abbe number for negative lenses are used. This is a desirable choice of glass materials for suppressing variations in chromatic aberration resulting from zooming or focusing. However, as seen in the Japanese Patent Laid-Open No. 63-34505, there is also proposed a method by which glass materials for fixed lens units are selected conversely to the above-mentioned to reduce the residual component of chromatic aberrations occurring in the zoom section.
Generally, zoom lenses of a negative lead form are characterized by the relative ease of widening the field angle and achieving predetermined back focus.
However, in order to achieve satisfactory optical performance over the whole range of magnifications and over the whole picture frame, it is necessary to appropriately set the optical power distribution and lens shapes of each lens unit.
If the optical power distribution and lens composition of each lens unit are inappropriate, the variation of aberration along with zooming will increase, making it difficult to achieve a high level of optical performance over the full magnification range.
Especially in a two-unit zoom lens in which a lens unit having negative optical power is in the leading position, the relative positions of the constituent lens units on the optical axis are uniquely determined by the zooming and the compensations for variations in the image position. As a result, it is impossible to control as desired the optical performance on the way of zooming from the wide angle end to the telephoto end.
Therefore, in order to improve the optical performance on the way of zooming, it is necessary to minimize the aberration variations of each lens unit in the process of zooming. This is accomplished by, for instance, by easing the optical power of each lens unit or increasing the number of lenses constituting each lens unit. However, these solutions involve the problem of increasing the overall lens length, making it difficult to increase the magnification and enhance the performance.
In order to solve these problems, there is proposed in the U.S. Pat. No. 5,570,233, a zoom lens which consists of, in the order from the object side, a first lens unit having positive optical power, a second lens unit having negative optical power, a third lens unit having positive optical power and a fourth lens unit having positive optical power, and varies the magnification by moving each lens unit. However, along with the development of image pickup devices, in the field of video cameras and digital cameras and the like of which further performance improvement is required, even greater enhancement of optical performance is called for.
Also, a zoom type proposed in the Japanese Patent Laid-Open No. 63-34505, which performing zooming with the lens unit on the object side and forms an image with the relay lens unit on the image side, is unsuitable for the shape of a camera which is provided with a barrel collapsing mechanism to shorten the overall length.